Chromium coating is widely used as a surface coating for different articles because of its high hardness value, attractive appearance and superior wear and corrosion resistance. Traditionally, Cr deposition is accomplished by electroplating from an electrolytic bath containing hexavalent Cr ions. The process is highly toxic in nature. Lots of efforts have been made to develop alternative coatings and coating processes to replace hexavalent Cr in electroplating. Among those alternative processes, trivalent Cr electroplating seems to be attractive due to its low cost, convenience of fabrication through the use of environmental friendly and non-toxic chemicals, and ability to produce a bright Cr deposit. However, an industrial scale process giving a hard and corrosion resistant Cr deposit through an aqueous trivalent chromium solution is still missing.
Many chromium plating processes of prior art are not capable of producing coatings with a Vickers microhardness value of 2000 HV or more. Further defects of the known chromium-based coatings are their inadequate wear and corrosion resistances. Chromium coating as such is very brittle in character. The number of cracks and micro-cracks in a chromium coating increases together with the thickness of the coating, thus impairing the corrosion resistance of the coating.
Deposition of nickel, either by electroless plating or electroplating, has also been proposed as an alternative to hard chrome. Drawbacks of nickel plating include deficiencies in hardness, friction coefficient and wear resistance. Nickel plating and chrome are not interchangeable coatings. The two have unique deposit properties and, therefore, each has its distinct applications.
It is well known in the art that the hardness of a chromium coating can be improved, to some extent, by thermal treatment. According to P. Benaben, An Overview of Hard Cromium Plating Using Trivalent Chromium Solutions, http://www.pfonline.com/articles/an-overview-of-hard-chromium-plating-using-trivalent-chromium-solutions, the microhardness of a chromium deposit as-plated is about 700-1000 HV100. By a heat treatment at 300-350° C. the microhardness of trivalent Cr can be increased up to about 1700-1800 HV100. At higher temperatures the hardness of the Cr deposit tends to decrease. Adhesion of a trivalent Cr layer is known to cause problems. The process chemistry of known trivalent Cr baths is often very complicated and hard to manage.
In patent document GB 921,977 a process for producing a nickel-chromium alloy coating on a metal base is disclosed. The process comprises applying a powdered alloy of nickel, chromium and phosphorus in an amount to provide from at least about 1 to about 4 grams of said fused alloy per square foot of fused coated surface. The base is then heated in a protective non-oxidizing atmosphere at a temperature and for a time sufficient to melt the powdered alloy. Thereby a continuous fusion coating of said alloy on the surface of said base is provided.
In patent document U.S. Pat. No. 5,232,469, a multi-layer coated diamond abrasive particles having improved wear performance are disclosed. The coating comprises a single homogenous, carbide forming metal primary layer, preferably of chromium, and at least one non-carbide forming secondary layer applied by electroless deposition, preferably comprised of nickel/phosphorus or cobalt/phosphorus.
The compound chromium-nickel-phosphate (CrNiP) is a ternary phosphide whose crystal structure has been studied. The production of CrNiP is known from studies concentrating on its crystal properties. In Stadnik et al. (Magnetic properties and 61Ni Mössbauer spectroscopy of the ternary phosphide CrNiP; J. Phys.: Condens. Matter 20 (2008) 285227), crystalline CrNiP was produced by mixing pure powders of Cr, Ni and P, sealing the mixture in an evacuated silica tube and heating at 873 K for 2 days. After this, the reaction product was quenched and subjected to a vacuum heat treatment at 1073 K for 2 days and then quenched. The ingot was pulverized, mixed well and heated at 1173 K for 7 days, after which the reaction product was quenched.
The hardness, friction coefficient, wear and corrosion resistance of known trivalent Cr coatings are not sufficient to satisfy the demands of industry. Apparently, there is a need for a chromium-based coating which is able to yield such utmost mechanical properties that enable replacement of hexavalent chromium baths.